1. Field of the Invention
This invention relates generally to a system and method for operating a fuel cell stack in response to loss of a cell voltage signal and, more particularly, to a system and method for operating a fuel cell stack in response to loss of a cell voltage signal that includes determining whether the cell was operating properly prior to the signal being lost.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack by serial coupling to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As a fuel cell stack ages, the performance of the individual cells in the stack degrade differently as a result of various factors. In addition, there are various stack operating conditions that cause the cells to operate differently. There are various causes of low performing cells, such as cell flooding, loss of catalyst, etc., some temporary and some permanent, some requiring maintenance, and some requiring stack replacement to exchange the low performing cells. Although the fuel cells are electrically coupled in series, the voltage of each cell when a load is coupled across the stack decreases differently where those cells that are low performing have lower voltages. Because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Further, since the cells are electrically coupled in series, each cell must produce the full stack current. If one cell is starved of reactants, especially hydrogen, the voltage for that cell will drop and undesirable side reactions could occur. Thus, it is necessary to separately monitor the voltages of the fuel cells in a stack to ensure that the voltages of the cells do not drop below a predetermined threshold voltage to prevent cell voltage polarity reversal, possibly causing permanent damage to the cell.
Cell voltage monitors or stack health monitors are used to measure the voltage of the fuel cells in the stack to look for behavior in the cells indicative of problems with the stack. The cell voltage monitor generally includes an electrical connection to each bipolar plate, or some number of bipolar plates, in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell. Therefore, a 400 cell stack may include 401 wires connected to the stack. If the cell voltage monitor fails, it is not possible to see a voltage drop and the system becomes unable to react by limiting power or changing system operating conditions. Continued operation with unhealthy cells can degrade the catalyst, especially on the anode, and potentially lead to cell shorting and internal hydrogen leaks.
The various connections to the fuel cell plates or other structures on the fuel cell stack may fail or otherwise be damaged where a signal for a particular cell or group of cells may be lost. In the event that the voltage signal from a cell or group of cells in the stack is lost, the fuel cell system needs to make some adjustments in system operation, possibly running the stack at a lower performance and efficiency than is desirable so that in the event that the cell or group of cells that is not being monitored does begin to lose voltage, the likelihood that serious damage to the stack could occur is reduced. Depending on the circumstances of the stack operating conditions, the system may be designed to provide severe power limitations in the event of loss of voltage signal from the cell to prevent stack damage.